a. Field of the Invention
This invention pertains to devices used to aid the pilot of an airplane in determining the point in the take-off roll of an airplane at which the pitch or attitude of the airplane safely may be rotated to begin take-off from the ground.
b. Description of the Prior Art
This invention is an improvement of the invention described in U.S. Pat. No. 4,594,592 ("592"). The specification of the "592" patent discloses a device which indicates when airplane rotation may be initiated to begin take-off. As pointed out in the "592" patent, air speed sensors and indicators have been used in the prior art to indicate when an airplane has attained a predetermined air speed that is deemed sufficient for safe rotation of the airplane for take-off. However, if part of the air speed is due to a headwind which decreases suddenly immediately after take-off, insufficient air speed may then remain for a safe continuation of the take-off. As a consequence, rather than depend only upon indicated air speed as an indication that a safe speed has been reached for rotation, a safer procedure is to use an air speed sensor combined with a ground speed sensor that determines the amount of the headwind and then adjusts the minimum air speed for take-off to higher values if the headwind exceeds a predetermined minimum.
The air speed of the airplane typically is measured by means of a Pitot tube located at an appropriate place on the fuselage of the airplane. The air speed indicated by such an instrument, which air speed is generally referred to as the "indicated air speed," differs from the actual, or true, air speed as a function of air density and compressible flow. The amount of the "lift" to the airplane that can be provided at take-off by the airflow over the airplane wings is a function of the dynamic air pressure. The dynamic air pressure is substantially the same as the pressure that is sensed by the Pitot tube and that is described in terms of an "indicated air speed." As a consequence the "indicated air speed", rather than the true air speed, normally is utilized for determining when take-off rotation may be initiated.
For a specific ambient air temperature and pressure, the ratio between indicated air speed and true air speed is a constant. This constant, however, is a function of both the ambient air temperature and pressure.
Prior to take-off the pilot of the airplane typically uses the temperature, loaded weight of the airplane and performance data for the airplane to determine from charts or other sources the minimum indicated air speed for safe airplane rotation to initiate take-off. In the "592" patent, the minimum indicated air speed, in effect, was reduced by an appropriate margin to determine the minimum safe ground speed for take-off rotation that would allow for a decrease in headwind just after take-off. As pointed out above, because of the dependence of indicated air speed upon ambient air temperature and pressure, the indicated air speed for take-off rotation may differ significantly from the corresponding true air speed required for take-off rotation. As a consequence, if the minimum actual or "true" ground speed for take-off rotation is determined simply in terms of the minimum indicated air speed less a specified margin of perhaps 5 to 10 knots, the actual or true minimum ground speed may differ from the true minimum air speed for take-off rotation by a substantially greater margin, thus providing less protection against a sudden decrease in headwind shortly after take-off.